1. Field of the Invention
The invention relates to a system for recognizing the intrusion of a conductive or semiconductive article into an area having an ambient electric field by detecting changes in the electric field. More specifically, the invention relates to such a system which is adapted to use existing mains wiring as the transmitting antenna.
2. Description of Prior Art
Systems of the above nature are known in the art and are generally used as alarm signals in household or in industrial or commercial establishments. Examples of such systems are described in U.S. Pat. Nos. 2,355,395, Rubenstein, Aug. 8, 1944; 4,155,078, Bowling et al, May 15, 1979 and 4,254,413, Mongeon, Mar. 3, 1981.
The known systems require separate, and sometimes complicated, transmitting antennas to provide a sufficient level of electric field in the environment. Such antennas can be inconvenient as they represent another element to be hidden, and they can also be costly.